In this Phase II proposal we will continue the development of a novel high-sensitivity direct-conversion x- ray detector that could revolutionize digital mammography because it will for the first time allow counting of individual photons leading to improved detection performance. Until now all commercial mammography systems have utilized integrating film or detectors. By combining a high-sensitivity detector material, mercuric iodide, with newly available CMOS technology photon-counting readouts, we expect to be able to achieve a significant breakthrough in x-ray detector performance. With the new technology we can realize improvement in the signal to noise ratio by essentially removing the dark current and readout circuitry noise from the signal. These factors are the specific limiting factors that determine the sensitivity of integrating mode detectors. Photon counting will reduce the dose delivered and improve the system throughput without compromising image quality. This is important in digital mammography screening due to the large number of healthy patients who are examined. We have reduced the technological risk of the Phase II work by our accomplishments during the Phase I work: We fabricated large area (10 cm x 10 cm) imaging charge-integrating CMOS readout chips which are chemically compatible with mercuric iodide, adapted our mercuric iodide film growth process for use with these chips, and created and tested HgI2/CMOS x-ray imagers. We also designed and manufactured specialized photon-counting ASICs for x-ray imaging applications. During Phase II we will adapt the ASIC design and implement it in the mercuric-iodide-compatible CMOS chips to produce a high-sensitivity photon-counting x-ray imager. We will take advantage of our design experience and newly-developed CMOS readout technology, allowing us to place a charge-sensitive preamplifier, shaping amplifier, dual discriminators, and digital event counter for each pixel in the array, all within each pixel's geometric area. Performance testing of detector panel prototypes will be performed at both DxRay and the University of California, Davis, Medical Center. We will widely market the device as an OEM component to mammography system manufacturers.